Blow Molding Of Polymeric Materials by Taylor A. Groll Padnos College of Engineering and Computing Grand Valley State University Laboratory Module 7 EGR 367 – Manufacturing Processes Instructor: Dr. P. N. Anyalebechi October 12, 2004T. Groll 1. Abstract The effects of barrel temperature, cycle time, and chemical composition of the plastic material on the uniformity of bottle wall thickness and the recovery of the blow molding process were examined using three different types of plastic and three different barrel temperature settings. Each type of plastic was blow molded at each of the three different barrel temperatures to determine its significance. The barrel temperature of 185 °C produced the greatest and most consistent bottle wall thickness as well as the greatest recovery for the three plastics used. Use of a 185 °C barrel temperature in the extrusion blow molding process would produce good parts for LDPE, HDPE, and santoprene plastics. 2. Introduction Blow molding is a process where by hollow objects are created from polymeric materials. The principle behind blow molding is identical to that of injection molding in that plastic pellets are melted and put into a mold to form a particular shape. With blow molding, a tube of molten called a “parison” is extruded out of the barrel of a blow molding machine. The parison is extruded down until the open end covers a small air nozzle. At this point, the two halves of the molded are closed on the parison and air is pumped into the mold cavity, forcing the molten plastic to conform to the inside features of the mold. Many common products are produced by blow molding including milk jugs, detergent bottles, garbage cans, plastic drink bottles, ice chests, chairs, squeeze toys, air ducts for automobiles, and chemical and gasoline tanks. The molds used to make blow molded objects are typically made out of aluminum. Mostly commodity and engineering grade thermoplastics are used in blow molding operations. The raw materials are generally in the form of pellets or granular compounds. Cycle times for blow molding processes are slower than those for injection molding. There are two common types of blow molding processes: extrusion blow molding and injection blow molding. The experiment outlined in this report was performed using an extrusion blow molding machine (Manufacturing Processes). Extrusion blow molding requires the extrusion of a hollow tube of plastic called a parison between two open mold halves. Clamping the two mold halves together seals off one end 2T. Groll of the parison and leaves the other end open. Compressed air is injected through the open end of the parison which forces the molten plastic against the walls of the mold cavity. The air injected into the mold also helps cool and solidify the plastic. Upon solidification of the plastic, the mold halves are opened and the part is removed. The extrusion blow molding process is typically used to make parts that do not have large changes in features such as long cylindrical bottles (Manufacturing Processes). Injection blow molding uses an injection molding process to form a molten plastic hollow tube called a preform. The usage of a preform produces less scrap because the preform does not extend below the blow mold. The hollow molten tube of plastic called a preform also makes it easier to produce accurate wall thickness in the parts produced. This is accomplished by varying the quantities of plastic present at different locations on the preform. If a bottle has a skinny top and a large bottom, the preform can be extruded to contain a larger amount of plastic near the bottom so that the overall wall thickness of the finished product is relatively uniform (Manufacturing Processes). 3. Experimental Procedure The blow molding machine was first inspected to ensure that all of the parts were in good working condition. The inspection consisted of a functional check of the barrel, plunger, nozzle, clamps, nozzle air supply, switch and the bottle mold. The proper closure of the bottle mold around the air supply nozzle was also checked, as well as the mating of the alignment pins with the other half of the mold. The heaters were then turned on via the power switch. The barrel temperature was initially set to 160 °C to allow it to warm up. The nozzle temperature was set to 165 °C and allowed to warm up. The nozzle temperature was set higher to keep the parison from melting as it was extruded from the nozzle and into the mold. A small amount of mold release was applied to the mold cavity. A scoop was used to fill the barrel with pellets of the low-density polyethylene (LDPE). High-density polyethylene (HDPE) and santoprene were also blow molded using the extrusion blow molding machine. The LDPE material was allowed approximately 5 minutes to melt in the barrel before producing the first part. To begin extrusion, the plunger handle was pulled down to extrude the parison from the blow molding nozzle. During extrusion, a scribe was used to keep the end of the parison from 3T. Groll closing and to guide the parison down onto the air supply nozzle in a manner so as to avoid the unnecessary stretching of it. Once the parison had sufficiently covered the air supply nozzle, the plunger handle was quickly released and the mold halves were clamped together before the plastic had a chance to cool or solidify. Care was taken to ensure that the mold halves were rapidly closed at the same time. Upon closure of the mold, the air supply was immediately switched on. After approximately 30 seconds, the air supply was turned off and the mold halves were carefully unclamped and opened. A scribe was used to gently pry the plastic part from the air supply nozzle. Two new bottles were made with a cycle time of 5 minutes and barrel temperatures of 145 °C and 185 °C, respectively. Unique features of the bottles were noted and each was photographed. A flash weight test was also performed on each of the bottles in addition to a wall thickness test. The wall thickness test was performed by first labeling four sides of the bottle, two on each side of the weld line. The bottle was then cut into two pieces. Using a micrometer, the wall thickness was measured at three locations on each bottle half from a known distance from the bottom of the bottle. The same procedures were then repeated for the HDPE and santoprene plastics.